The cost of aluminum production can be reduced by substituting inert anodes for the carbon anodes now used in most commercial electrolytic cells. Inert anodes are dimensionally stable because they are not consumed during aluminum production. Using a dimensionally stable inert anode together with a wettable cathode allows more efficient cell designs, lower current densities and a shorter anode-cathode distance, with resulting energy savings.
One problem associated with inert anodes is that they may contain metal oxides having some solubility in molten fluoride salt baths. In order to reduce corrosion of the inert anodes, cells containing them should be operated at temperatures below the normal Hall cell operating range (approximately 948.degree. to 972.degree. C.). However, reduced temperature operation also poses some problems, including difficulty in maintaining an electrolyte saturated with alumina, solidification of electrolyte in the cell (sludging) and floating aluminum. In addition, some types of inert anodes tend to form resistive layers at lower operating temperatures.
In order to achieve low corrosion rates on the inert anodes, the alumina concentration must be maintained near saturation but without a high bath velocity near the anodes and without sludging of the cell. Some electrolyte circulation is required to dissolve the alumina, but circulation can also accelerate anode wear by circulating aluminum droplets. We have discovered that these problems can be avoided by providing a highly agitated alumina feed area, separated from the electrodes in order to improve alumina dissolution without also increasing corrosion of the inert anodes.
An important objective of the present invention is to provide an electrolytic cell having an inert anode and a slanted roof that diverts oxygen bubbles generated at the anode toward an upcomer channel wherein a metal oxide is dissolved.
A related objective of the invention is to provide a process for producing a metal in a cell having a molten salt bath, wherein a portion of the molten salt bath in an upcomer channel is agitated without any need for stirrers, pumps, or other conventional agitating means.
Additional objectives and advantages of our invention will become apparent to persons skilled in the art from the following detailed description.